a) Field of the Invention
The present invention relates to a surface light source device which is to be used as a back light for liquid crystal display units, and more specifically to a surface light source device which uses a transparent panel member or a reflecting panel member.
b) Description of the Prior Art
The conventional surface light source device which uses a transparent panel has a composition as illustrated in FIG. 1. In this drawing, the reference numeral 1 represents a light source, the reference numeral 2 designates a transparent panel, the reference numeral 3 denotes a diffusing panel and the reference numeral 4 represents a reflecting surface. The light source 1 used in this surface light source device is a linear light source which is, for example, a cold cathod ray tube and extends in the direction perpendicular to the paper surface. The transparent panel 2 is made of a glass material or a transparent plastic material. As a material for the transparent panel 2, it is generally known to select a plastic material such as acrylic resin which is light in weight thereof and available at a low cost. The transparent panel 2 has a form of a plane parallel plate which has thickness of t and a rectangular or square surface. A light beam emitted from the linear light source 1 enters through an end surface of incidence 2a into the transparent panel 2 and travels through the transparent panel 2 as represented by a ray 10 in FIG. 1. During this travel, portions of the light beam emerge from the transparent panel 2 (upward in FIG. 1) and pass through the diffusing panel 3 to produce diffused rays. The surface light source device produces the diffused rays which are diverging from various points on the surface of the diffusing panel (the upper surface shown in FIG. 1) as described above. The ray 10 shown in FIG. 1 is diverged from a point A on the diffusing panel 3. In a case where the diffusing panel 3 should not be used, a ray which travels in a direction having an angle of .gamma. indicated by an arrow has the highest brightness. The angle .gamma. has a value which is different dependently on refractive indices of materials selected for the transparent panel 2. Experiments effected by selecting an acrylic resin having a refractive index of 1.49 indicated results that the angle .gamma. has a value of approximately 75.degree.. When the diffusing panel 3 is placed over the transparent panel 2 so that rays having passed through the diffusing panel 3 are diffused, deflection of the rays is corrected a little due to the diffusion of the rays, but brightness in the direction perpendicular to the transparent panel is not enhanced so remarkably. When a liquid crystal display panel, for example, is illuminated by using the surface light source device which uses the diffusing panel 3 placed over the transparent panel 2, the brightness of the diffused rays is low in a direction for observation, i.e., in a direction having the angle .gamma.=0.degree., whereas brightness of the diffused rays is the highest in an oblique direction having an angle .gamma..apprxeq.75.degree. which is deviated from the direction for observation. Accordingly, an observer of the liquid crystal display panel is undesirably obliged to observe an image which is relatively dark.
As another conventional example of the surface light source device which uses the transparent panel, there is also known the surface light source device illustrated in FIG. 2. In this conventional example, two linear light sources 1 are disposed beside both end surfaces of a transparent panel 5. The two light sources 1 are adopted for obtaining a surface light source device which can provide diffused rays having higher brightness.
Further, the transparent panel 7 of the surface light source device illustrated in FIG. 2 has a thinned central portion (the portion farthest from the light sources disposed on the right and left sides) and a top surface 7b which is inclined.
The surface light source device illustrated in FIG. 2 is configured so that light beams emitted from the right side and left side light sources 1 enter through end surfaces of incidence 7a of the transparent panel 7, and emerge out of a diffusing panel 3 so as to produce diffused rays. In the case of the surface light source device which uses the inclined top surface 7b, rays emerging from the transparent panel 7, for example rays 10 and 11, have the highest brightness at an inclination angle which is a little smaller, or the angle .gamma. has a value of approximately 60.degree.. However, the surface light source device illustrated in FIG. 2 cannot provide the diffused rays which have sufficient brightness in the direction for observation and causes the observation of relatively dark images.
In addition, there are known transparent panels each of which has thickness progressively thinned along a curve from an end surface of incidence as well as those each of which has thickness progressively thinned along curves from a plurality of end surfaces of incidence. There are also known transparent panels each of which is combined with a plurality of light sources and has uniform thickness.